In motor vehicles, which are equipped with catalytic converters, driving the vehicle until the tank is completely empty can lead to malfunctions of the catalytic converter and to other disturbances of the engine. When the content of the fuel tank goes to empty, the fuel pump sporadically pumps vapor instead of liquid. The required fuel metering to one or several cylinders of the engine can drop to such an extent that combustion misfires occur. The uncombusted fuel is then afterburned together with oxygen in the catalytic converter whereby the temperature of the catalytic converter increases. The temperature increase can be so great that permanent damage can occur to the catalytic converter.
The regulations of the California environmental authority (CARB) as well as United States federal regulations require that an automobile manufacturer must replace a defective catalytic converter within the first 100,000 miles. In addition, if, during the warranty period, a specific percentage of catalytic converters becomes defective, the California environmental authority even requires a recall. Such measures are most disadvantageous to automobile manufacturers since they are associated with considerable difficulty and expense. Furthermore, the reputation of the manufacturer can suffer damage in the marketplace.
In view of the above, there is a very substantial interest on the part of the automobile manufacturers to distinguish catalytic converter malfunctions, which are caused by the conduct of the driver (such as unpermittingly driving until the fuel tank is empty), from malfunctions caused by equipment failure.
A further requirement of the California environmental authority since mid 1994 requires that a diagnosis be made with on-board means to check the operability of the vehicle systems. These requirements include, inter alia, that a check of the operation of the catalytic converter be made with on-board means. As soon as a defect of a technical nature occurs, this defect must be signalized to the driver of the vehicle and stored in a memory in order to later, via a read-out of the memory, be able to make a statement as to whether the defect is of a technical nature or whether the defect was caused by improper conduct on the part of the driver operating the vehicle.
As mentioned above, for an almost empty tank, combustion misfires occur which cause crankshaft rpm fluctuations, that is, disturbances of the crankshaft rpm.
U.S. Pat. No. 5,513,521 discloses a method for detecting combustion misfires which permits combustion misfires and disturbances of the crankshaft rpm to be distinguished from each other. The disturbances of the crankshaft rpm are based on reactions of the vehicle drive train, for example, because of extreme roadway unevenness, because of jolting vehicle operation or because of a change of tire slip and tire adherence to the roadway.
This method permits precise differentiation of the above-mentioned disturbances of the crankshaft rpm from disturbances which are caused by combustion misfires. However, it is not possible to make a statement as to whether the combustion misfires or the disturbances of the crankshaft rpm are caused by an improper operation of the vehicle (for example, an empty tank) which can be attributed to the operator of the vehicle.